Tuning system for a multi-band television receiver

ABSTRACT

A television tuning system includes UHF and VHF signal paths each tuned by a tuning voltage and coupled to a common signal path by a diplexer. So that UHF signals will be attenuated so to not cause interferences with a VHF channel signal which is selected to be viewed, the UHF filter is detuned when a VHF channel is selected. Detuning the UHF filter is accomplished by modifying the tuning voltage for the UHF filter responsive to the VHF band-switching voltage.

This invention relates to tuning systems for television receivers and,in particular, to those in which tuning of a tuned signal path ismodified for certain conditions.

Television (TV) tuning systems select desired ones of many TV radiofrequency (RF) signals at different frequencies received from broadcastor cable TV signal sources in order to produce an intermediate frequency(IF) signal from which display and sound information is derived. In adouble-conversion tuning system, two frequency conversions (shifts) areperformed to translate the received RF signal first to a first IF signaland then to a second and final IF signal.

It is not uncommon for signals of different channels to interfere witheach other causing degradation of the TV picture and/or sound ultimatelyreproduced. The degree to which such interference is generated dependsupon the frequency selective characteristics of filters, the operationof the mixer and local oscillator, and the selection of the intermediatefrequency ranges.

Consider, for example, the selection of VHF channel 5 which has an RFpicture carrier at 77.25 MHz by a double-conversion tuning system havinga first IF passband surrounding a first picture carrier frequency of415.75 MHz. A local oscillator frequency of 493 MHz is developed so thata first mixer converts the 77.25 MHz RF picture carrier to the 415.75MHz. If a signal at UHF channel 31 having an RF picture carrier at573.25 MHz also becomes mixed with the local oscillator signal, a signalat the difference frequency 80.25 MHz may be produced. Since 80.25 MHzis within the frequency band of VHF channel 5, (i.e., 76 to 82 MHz), aninterference condition may be produced. In similar fashion, there existUHF RF carriers which can produce interference conditions for each ofthe low-band VHF channels 2 through 13, as well as many of the cable TVchannels.

Conventionally, the generation of interfering signals can be inhibitedby placing switches, such as PIN diodes or transistor switches, in thesignal path between the source of RF signals and the mixer to therebyprevent potentially interfering signals in a non-selected band fromreaching the mixer. At UHF frequencies, however, signal "leakages"through or around such switches when they are supposed to benon-conductive, such as by "stray" capacitances, can be of sufficientstrength to cause undesirable interference. The present inventionprovides substantial attenuation of unwanted signals to ensure that anyinterference developed will be insubstantial and therefore notdiscernible in the TV picture or sound.

To that end, the tuning system of the present invention comprises firstand second signal paths corresponding to first and second tuning bandsand having frequency selective characteristics controlled in response toa tuning signal developed by a tuning control device. Signals from thefirst and second signal paths are combined onto a common signal path.The tuning control device also develops a band-indicative signal formodifying the tuning signal applied to the first signal path when achannel in the second band is selected.

In the drawings:

FIG. 1 is a schematic diagram in block diagram form of a tuning systemincluding an embodiment of the present invention;

FIG. 2 shows graphical representations of various amplitude versusfrequency characteristics associated with the embodiment of FIG. 1; and

FIGS. 3 and 4 are schematic diagrams of circuits including embodimentsof the present invention useful in the tuning system of FIG. 1.

In the double conversion tuning system of FIG. 1, television signalsreceived at UHF antenna input 10, at VHF antenna input 30A and at CATVinput 30B are coupled to diplexer 20 as will be described below. In theUnited States, those television signals comprise the channel numbers andreside in the frequency bands indicated in the following table.

                  TABLE 1                                                         ______________________________________                                                           Frequency                                                                     Range      Channel                                         TV Band            (megaHertz)                                                                              Designations                                    ______________________________________                                        Low VHF Broadcast (L-VHF)                                                                        54 to 88   2 to 6                                          Midband Cable (MB-CATV)                                                                          90 to 174  A-5 to I                                        High VHF Broadcast (H-VHF)                                                                       174 to 216 7 to 13                                         Superband Cable (SB-CATV)                                                                        216 to 402 J to W + 17                                     UHF Broadcast (UHF)                                                                              470 to 890 14 to 83                                        ______________________________________                                    

Each channel is alloted about 6 MHz of bandwidth in the frequencyspectrum and has a picture carrier at a frequency 1.25 MHz higher thanthe frequency at the lower limit of its assigned channel bandwidth.Where specific frequencies are referred to in the followingdescriptions, that frequency will correspond to the frequency of thepicture carrier (PIX) of the selected television channel.

The frequency spectrum for channel frequencies "f" in the various TVfrequency bands in the United States is shown in FIG. 2(a). For theL-VHF band 202, H-VHF band 206 and UHF band 210, the amplitude ofreceived signals is shown as a plurality of levels indicating thatbroadcast signals can vary in strength over a wide range, for example,between 10 microvolts and 100 millivolts. Received CATV signals, on theother hand, exhibit a much smaller variation in signal strength,typically between 1 and 6 millivolts, as illustrated for the MB-CATVband 204 and the SB-CATV band 208.

FIG. 2(b) defines the low-band, high-band and UHF-band of radiofrequencies (RF) associated with filters 44, 36 and 14, respectively, ofFIG. 1 to be described below. The picture carrier of the firstintermediate frequency (IF) signal is selected to be at 415.75 MHz,which is between the SB-CATV and the UHF-TV bands. The picture carrierof the second IF signal is at the standard frequency of 45.75 MHz usedin the United States. The bandwidth of the desired TV signal at each IFfrequency is approximately 6 MHz and is centered on about 414 MHz forthe first IF and 44 MHz for the second IF.

It is understood that while the present invention is described in termsof the various broadcast and cable bands presently used in the UnitedStates, the invention is not limited to that arrangement. For example,off-the-air broadcast band signals could in fact be supplied from acable TV signal source and, in addition, be in TV frequency bandsemployed elsewhere, such as Europe or Japan.

As shown in FIG. 1, when the TV channel selected is within the UHF band,it is coupled from UHF antenna 10 to a first input terminal of diplexer20 via the UHF-band frequency selective tunable filter 14. Filter 14receives tuning potential VT at connection 14C so that it is tuned topreferentially pass between its input 14A and its output 14B signals atthe frequencies corresponding to that of the selected TV channel.

UHF filter 14 exhibits a tunable low-pass frequency selectivecharacteristic in response to tuning voltage VT. It exhibits attenuationof signals having frequencies below that of the selected channel andexhibiting relatively greater attenuation at frequencies higher thanthat of the selected channel. Filter 14 exhibits a peak amplituderesponse at about the frequency of the selected channel (i.e. a peakpassband having minimum attenuation). The bandwidth of that peak isabout 25 MHz when tuning near UHF channel 14 and increases somewhat toabout 40 MHz when tuning near UHF channel 83, for the specificembodiment of filter 14 described below with respect to FIG. 3.

IF Trap 12 reduces the level of any signals at frequencies close to thefirst IF received by UHF antenna 10. This is desirable because the415.75 MHz first IF frequency is near the low end of the UHF band. Thoseundesirable signals could either be externally developed, e.g. radio orradar signals, or could be radiated or conducted from the first IFsection. Trap 12 suppresses unwanted signals near the first IF frequencywhich might otherwise be applied to the IF circuits. For this purpose,trap 12 attenuates signals between 411 and 417 MHz.

UHF amplifier 16 couples signals between output 14B of filter 14 and afirst input of diplexer 20. Amplifier 16 exhibits about 14-15 dB of gainover the UHF frequency range and has input and output impedances ofabout 50 ohms to match the impedances of filter 14 and diplexer 20.Amplifier 16 is operative only when a channel in the UHF band has beenselected because its operating voltage, bandswitch voltage VB3 (about 18volts), is present only when channels in the UHF frequency band havebeen selected, as indicated by level 260 of FIG. 2(f). Because amplifier16 is not powered when signals in other than the UHF band are selectedto be received, it provides attenuation in the path between antenna 10and diplexer 20 at such times.

TV signals in the VHF and CATV bands (54 to 402 MHz) are partitionedinto low and high tuning bands because they span more than aseven-to-one range of frequencies. Tuning over a range greater thanthree-to-one is impractical owing to the limited range ofvoltage-variable capacitance diodes. Accordingly, the tuning apparatusof FIG. 1 for the VHF and CATV bands is partitioned to tune in low andhigh tuning bands separated at a frequency within the MB-CATV band, i.e.about 150 MHz, as shown in FIG. 2(b). As a result each of the high andlow tuning bands includes signals having less than a three-to-one rangeof frequencies.

TV signals in the VHF and CATV frequency bands are coupled to diplexer20 in FIG. 1 as follows. Switch S1A can be switched to position BC-A toapply signals from VHF antenna 30A to the input of filter 32 or can beconnected to position CA-A to apply CATV signals from input terminal 30Bto filter 32. Filter 32 is a high-pass filter which attenuates signalsat frequencies below about 40 MHz which is slightly less than the lowestfrequency to be received (i.e., VHF channel 2 at 54 to 60 MHz). Filter32 passes signals in both the low (54 to 150 MHz) and high (150 to 402MHz) bands. If the selected channel is in the high-band, then VB2 isapplied to switches 34 and 38 to render them conductive (closed) tothereby couple high-band filter 36 between filter 32 and VHF amplifier40. If the selected channel is in the low-band, however, then VB1 isapplied to switches 42 and 46 to render them conductive to connectlow-band filter 44 between filter 32 and VHF amplifier 40.

High-band tunable filter 36 exhibits a tunable high-pass frequencyselective characteristic in response to tuning voltage VT. It exhibitsgreater attenuation of signals having frequencies below that of theselected high-band channel then of signals having frequencies above thatof the selected channel. Minimum attenuation is exhibited in a peakpassband which includes the frequency of the selected channel. As aresult, filter 36 not only selects the RF carrier for the selectedchannel but also tends to reject signals at lower frequencies,particularly those signals in the low band.

Low-band tunable filter 44 exhibits a tunable low-pass frequencyselective characteristic in response to VT similar to that describedabove in relation to UHF filter 14 except that the bandwidth of its peakamplitude response passband is allowed to increase to a substantiallygreater degree as higher frequency channels are selected. Filter 44 notonly selects the RF carrier of the selected channel but also tends toreject signals at higher frequencies, particularly those signals in thehigh band and at the first IF frequency.

VHF amplifier 40 in FIG. 1 couples signals from filter 36 or 40 to asecond input of diplexer 20. Operating potential VB12 is applied to VHFamplifier 40 whenever a channel within the low or high tuning bands isselected, but VB12 is not applied when a channel in the UHF band isselected. The VHF and CATV signal paths are thus disconnected fromdiplexer 20 when a UHF channel is selected. Operating potential VB12 isdeveloped from bandswitch voltages VB1 and VB2 by a diode "OR" circuitincluding diodes D12 and D14.

Diplexer 20 of FIG. 1 receives RF signals from the UHF band signal pathat a first input connection and receives RF signals from the VHF andCATV band signal paths at a second input connection and couples thereceived RF signals to a common signal path at its output connection.Diplexer 20 can satisfactorily be a passive-element signal combiner.

Mixer 50 receives RF signals from the output of diplexer 20 and localoscillator frequency signals from amplifier 52 and translates the RFsignals to an IF signal having a picture carrier at the first IFfrequency of 415.75 MHz.

A tunable voltage controlled local oscillator arrangement (VCO) 56develops the local oscillator frequency signal for each of the threetuning bands. The VCO frequency is responsive to tuning voltage VT so asto track the tuning of the appropriate one of band filters 14, 36 and44. The range of VCO frequencies are indicated in the following table.

                  TABLE 2                                                         ______________________________________                                                                   Local Oscillator                                   Band     Channel Number    Frequency (MHz)                                    ______________________________________                                        Low-Band 2 to 6 (L-VHF)    471 to 499                                                  A-5 to E (MB-CATV)                                                                              507 to 561                                         High-Band                                                                              F to I (MB-CATV)  567 to 585                                                  7 to 13 (H-VHF)   591 to 627                                                  J to W + 17 (SB-CATV)                                                                           633 to 813                                         UHF-Band 14 to 83 (UHF)    887 to 1301                                        ______________________________________                                    

Amplifier 52 amplifies the signal from VCO 56 so that the RF signalsfrom diplexer 20 can also be of greater relative strength withoutintroducing additional distortion in mixer 50.

The first IF signal from mixer 50 is then amplified by tuned IFamplifier 60. Amplifier 60 includes a two-section input filter tuned tothe 415.75 MHz first IF picture carrier frequency and having about 12MHz bandwidth, and a three-section output filter also tuned to 415.75MHz and having about 10 MHz bandwidth. The centers of the pass bands ofthese filters is, in fact, at about 414 MHz. The amplified IF signalfrom amplifier 60 is then mixed with a 370 MHz frequency signal fromsecond local oscillator 64 by mixer 62 to produce the second IF signalhaving a picture carrier at 45.75 MHz. The second IF signal is coupledto IF output 68 via IF filter 66.

Tuning control 70 responds to selection of a channel to develop tuningpotential VT and bandswitch potentials VB1, VB2 and VB3. Tuningpotential VT, shown in FIG. 2(c), typically varies between a low levelof about 1.5 volts, indicated by phantom line 220, and a higher level ofabout 24 volts, indicated by phantom line 222. When the selected channelis in the low tuning band, VT tends toward a low value at point 224 whenVHF channel 2 is selected and tends toward a high value at point 226when MB-CATV channel E is selected. When the selected channel is in thehigh tuning band, VT tends toward a low value at point 228 when MB-CATVchannel F is selected and tends toward a high value at point 230 whenSB-CATV channel W+17 is selected. Similarly, VT tends toward a low valueat point 232 when UHF channel 14 is selected and toward a high value atpoint 234 when UHF channel 83 is selected.

Bandswitch signals VB1, VB2 and VB3 are at a high level of about 18volts are indicated by characteristics 240, 250 and 260 of FIGS. 2(d),2(e) and 2(f) only when a channel in the band to which they correspondhas been selected, and are at zero volts when a channel outside thatparticular band is selected. U.S. patent application Ser. No. 271,742,entitled A PHASE-LOCKED LOOP TUNING SYSTEM INCLUDING A PRESCALERCONDITIONED TO OSCILLATE AT AN OUT-OF-BAND FREQUENCY, filed by D. J.Carlson et al. on June 5, 1981, and assigned to the same assignee as isthe present invention, is incorporated herein by reference for thepurpose of describing a tuning control device suitable for developingtuning and bandswitch potentials of the sort developed by control 70.

UHF band filter control 15 receives band signal VB12 which is at a highlevel whenever the selected channel is in the high or low band. In thatcase, the selected low band RF signal is passed by one of band filters36 and 44. UHF RF signals should not be coupled to diplexer 20 at suchtimes and are attenuated by UHF amplifier 16 being in an unpoweredcondition since UHF band signal VB3 is at a low level. However, strongUHF RF signals can "leak" through around or through amplifier 16 owingto unavoidable parasitic capacitive coupling paths inherent in itsphysical arrangement. These undesired UHF RF signals can be mixed withthe signal from VCO 56 in mixer 50 to become interfacing signals at theIF frequency as previously described.

Such undesired UHF RF signals are not ordinarily (i.e. without filtercontrol 15) attenuated by filter 14 which is tuned to track the tuningof band filters 36 and 44 because all three filters are tuned by thesame tuning voltage VT. To reduce interfering signals to levels whichare not discernible to the TV viewer, filter control 15 responds to bandsignal VB12 to modify the tuning voltage VT within band filter 14 viaconnection 14D. Filter 14 is thereby detuned so as to attenuate UHFsignals passing through it to UHF amplifier 16. As is explained below,detuning of filter 14 is accomplished by causing its tuned circuits toreceive modified tuning voltage so as to be tuned to a frequencysubstantially different from the frequency of the potentiallyinterfering channel.

UHF-band filter 14, shown in FIG. 3, is a double-tuned, low-pass filterwith "high-side inductive coupling" provided between its input 14A andits output 14B by the serial connection of inductors L402, L406, L408,L410 and L414. Capacitor C408 serves as a DC blocking capacitor withnegligible AC impedance at UHF frequencies. Inductors L404 and L406serve as a tapped inductor configuration for maintaining the impedanceat input 14A at about 50 ohms. Similarly, inductors L410 and L412 serveas a tapped inductor configuration for maintaining the impedance atoutput 14B at about 50 ohms. Input and output inductors L402 and L414aid in maintaining a substantially constant bandwidth over the broadtuning range of filter 14. Capacitor C404, which may comprisecapacitance associated with conductors on a printed-circuit board, is inparallel with inductor L408. C404 and L408 are selected to resonate atabout 1,000 MHz. Variable frequency tuning is provided by variablecapacitance (varactor) diodes CD42 and CD44 which are respectivelyconnected from the ends of the L408-C404 tuned circuit to ground viacoupling capacitors C402 and C406. C402 and C406 exhibit a very lowimpedance at the frequencies of the UHF television signals passed byfilter 14. The anodes of CD42 and CD44 are d.c. coupled to groundpotential through inductors L404, L406, L408, L410 and L412.

Tuning potential VT is applied at terminal 14C to vary the capacitanceof diodes CD42 and CD44 through isolation resistors R402, R404 and R406.VT can vary between about 1.5 and 24 volts for UHF channels 14-83 asindicated in FIG. 2(c). Because negligible d.c. currents are conductedby capacitance diodes CD42 and CD44, there is substantially no d.c.voltage drop developed across any of R402, R404 and R406 so thatessentially the full magnitude of tuning voltage VT is applied to thecathodes of CD42 and CD44.

Filter control circuit 15 modifies the voltages applied to CD42 and CD44to detune filter 14 when the selected channel is not in the UHF band asfollows. Band indicating voltage VB12 is then at a high level of about+18 volts to apply forward bias to the base of switch transistor TS viaresistors R410 and R412. TS is thereby rendered conductive to make aconductive connection between the cathode of CD44 and ground potentialincluding the collector-emitter path of TS and connection 14D. As aresult, resistor R406 and TS now serve as a voltage divider for VTbetween connection 14C and ground, thereby providing substantially thesame modified tuning potential at the respective cathodes of CD42 andCD44.

Because the voltage across each of CD42 and CD44 is substantiallyreduced, each exhibits a much larger capacitance thereby substantiallyreducing the resonant frequencies of the respective circuits in whichthey are coupled. Since those voltages are reduced by about the sameamount, those respective resonant circuits will become tuned to a lowerfrequency thereby making the peak passband of filter 14 shift to afrequency near or below the lowest frequency in the UHF TV band.

In the modification shown in FIG. 4, tuning voltage VT is applied to thejunction of resistors R402' and R406' in filter 14' by operationalamplifiers A1 and A2 as follows. VT is amplified by the invertingamplifier including A1 which has a gain determined by the ratio of theresistance of resistor R422 to that of resistor R420, which gain mayconveniently be minus unity. The inverting amplifier including A2amplifies the output voltage from A1 by a factor determined by the ratioof the resistance of variable resistor R432 to that of resistor R430.Resistor R432 is adjustable so that the gain of A2 can be adjusted to asuitable value. For example, the gain provided by A2 can be minus fivewhere the ratio of VT' to VT is one-fifth.

Amplifiers A1 and A2 receive biasing voltage +V/2 through resistors R424and R434, respectively, at their non-inverting (+) input terminals.Voltage +V/2 is conveniently made about one-half of supply voltage +Vwhich is the operating potential applied to A2, with respect to groundpotential.

Filter control 15' applies supply voltage +V to amplifier A1 when thechannel selected is in the UHF band to produce operation of A1 and A2 asdescribed above. To that end, UHF bandswitch voltage VB3 is substitutedfor VB12 and is applied to the base of switch transistor TS' throughresistors R410' and R412' to make TS' conductive. Control 15' causesdetuning of filter 14' when a VHF channel is selected because VB3becomes substantially zero volts causing TS' to become non-conductive toremove operating voltage from A1. Without operating voltage applied toA1, its output voltage is pulled towards ground potential by pull-downresistor R426. This modifies the tuning voltage applied to the junctionof R402' and R406' which is caused to increase towards supply voltage +Vby the operation of amplifier A2. As a result, the capacitances of CD42and CD44 are reduced substantially whereby filter 14' is caused to beretuned to a frequency higher than the highest frequency in the UHF TVband. Therefore, filter 14' attenuates UHF TV signals which otherwisemight cause interference.

It is noted that the present invention eliminates sources of potentialinterference other than that previously described herein. For example,where VCO 56 also develops a component signal at the second harmonic ofthe desired local oscillator frequency for the selected VHF TV channel,a signal from an unselected UHF channel can be shifted into the first IFsignal band by mixer 50 responding to the second harmonic signal. Onesuch case is where VHF channel 4 is selected. The second harmonic (966MHz) of the local oscillator 56 frequency (483 MHz) can shift a signalon UHF channel 27 (549.25 MHz PIX) to 416.75 MHz which is unacceptablyclose to the 415.75 MHz first IF signal frequency.

Modifications of the described embodiments are contemplated to be withinthe scope of the present invention which is limited only by the claimsfollowing. For example, it is also satisfactory that transistor TS inFIG. 3 or resistor R426 in FIG. 4 be connected so that the tuningvoltage, when modified, becomes larger in magnitude so to shift the peakpassband of filter 14 to a much higher frequency.

In addition, it is also satisfactory that a resistor be inserted in thecollector circuit of switch transistor TS to the junction of resistorsR402 and R404, as illustrated in FIG. 3 by resistor R414 (shown inphantom). Where the dashed line outlining filter 14 represents a radiofrequency shield, the embodiments described herein beneficially minimizethe number of feedthrough capacitors (not shown) needed to makeconnections through the shield.

What is claimed is:
 1. A tuning system for a television receivercomprising:first and second signal paths for receiving radio frequencysignals disposed in respective first and second frequency bands, each ofsaid signal paths including filter means for providing frequencyselective characteristics controllable in response to a tuning controlsignal; tuning control means for developing said tuning control signalhaving a magnitude to select one of said radio frequency signals withinsaid first and second frequency bands and for developing a band signalindicative of the one of said first and second frequency bands whichincludes said selected radio frequency signal; combining means forcombining signals from said first and second signal paths onto a commonsignal path; means for applying said tuning control signal to saidfilter means of said first and second signal paths; and means coupled tosaid first signal path and responsive to said band signal for modifyingthe magnitude of said tuning control signal applied to said filter meansof said first signal path relative to that applied to said filter meansof said second signal path when said selected radio frequency signal isincluded in said second frequency band.
 2. The tuning system of claim 1further comprising:oscillator means for developing an oscillator signalhaving a frequency determined in response to said tuning signal; mixingmeans coupled to said common signal path for heterodyning said combinedradio frequency signals thereon with said oscillator signal to develop aheterodyned signal; and said tuning control means controlling saidtuning control signal to determine the frequency of said oscillatorsignal so that said heterodyned signal has a predetermined frequency. 3.The tuning system of claim 1 wherein:said filter means of said firstsignal path includes a variable capacitance diode to which said tuningcontrol signal is applied; said means for applying includes a resistancethrough which said tuning control signal is applied to said capacitancediode; and said means for modifying includes means connected to acircuit point between said resistance and said capacitance diode forchanging the potential thereat in response to said band signal.
 4. Thetuning system of claim 3 wherein said means for changing the potentialincludes switch means for selectively making a conductive connection toa source of potential in response to said band signal.
 5. The tuningsystem of claim 4 wherein said switch means includes a transistor havingan output electrode coupled to said circuit point, having a commonelectrode connected to said source of potential, and having an inputelectrode to which said band signal is applied for causing saidtransistor to be conductive between its output and common electrodes. 6.The tuning system of claim 4 wherein:said means for applying furtherincludes amplifying means having a supply terminal for receiving anoperating potential thereat, and having input and output terminalsrespectively for receiving said tuning control signal and for applying asignal responsive to said tuning control signal to said capacitancediode, and said switch means is connected for applying said operatingpotential to the supply terminal of said amplifying means.
 7. The tuningsystem of claim 1 wherein:said filter means of said first signal pathincludes first and second resonant circuits, said first and seondresonant circuits respectively including first and second variablecapacitance diodes to which said tuning control signal is applied; saidmeans for applying includes at least one resistance through which saidtuning control signal is applied to said first and second capacitancediodes; and said means for modifying includes means connected to acircuit point between said resistance and said first and secondcapacitance diodes for changing the potential thereat.
 8. The tuningsystem of claim 7 wherein said means for modifying changes therespective potentials at said first and second capacitance diodes inlike sense.
 9. A tuning system for a television receivercomprising:first and second signal paths for receiving radio frequencysignals disposed in respective first and second frequency bands, each ofsaid signal paths including filter means for providing frequencyselective characteristics controllable in response to a tuning controlsignal; oscillator means for developing an oscillator signal having afrequency determined in response to said tuning control signal; mixingmeans coupled to said first and second signal paths and to saidoscillator means for heterodyning said radio frequency signals and saidoscillator signal to develop a heterodyned signal at a predeterminedfrequency; tuning control means developing said tuning control signalhaving a magnitude for controlling the frequency selectivecharacteristics of said filter means of said first and second signalpaths to track the frequency signals within said first and secondfrequency bands, and for developing a band signal indicative of the oneof said first and second frequency bands in which said selected radiofrequency signal is included; means for applying said tuning controlsignal to said oscillator means and to said filter means of said firstand second signal paths; and means coupled to said filter means of firstsignal path for modifying the magnitude of said tuning control signalapplied thereto to untrack said frequency selective characteristicsthereof and the frequency of said oscillator signal in response to saidband signal when said selected radio frequency signal is included insaid second frequency band.
 10. The tuning system of claim 9wherein:said filter means of said first signal path includes a variablecapacitance diode to which said tuning control signal is applied; saidmeans for applying includes a resistance through which said tuningcontrol signal is applied to said capacitance diode; and said means formodifying includes means connected to a circuit point between saidresistance and said capacitance diode for changing the potential thereatin response to said band signal.